Water, pipe scale, and sludge contaminated with naturally occurring radioactive materials (NORM) are wastes that often accompany the recovery of oil and gas from subterranean reservoirs. Most of the NORM-contaminated water is currently disposed of through injection back into the reservoirs from which it came. Unfortunately, solid NORM wastes, like scale and sludge, require extensive processing before disposal through underground injection into permeable rocks or land spreading can be accomplished. Because of the need to reduce the high costs involved in the conventional disposal of solid NORM wastes, it has been proposed that such be injected into salt caverns.
Salt caverns are typically created by injecting fresh water into subterranean salt formations and withdrawing the resulting brine. This process is referred to as solution mining. Over time, numerous salt caverns have been solution mined by the petroleum industry for use in storing hydrocarbons and for disposing nonhazardous oilfield wastes (NOW). To date, salt caverns have not been used to dispose of NORM wastes due to concerns that they may leak radioactive materials into surrounding rocks and, perhaps, into fresh water aquifers.
It has been noted that the release of NORM wastes from salt caverns could result from one of five scenarios: (1) inadvertent intrusion; (2) failure of the cavern seal; (3) leakage through cracks; (4) leakage through interbeds of permeable material; and, (5) a partial cavern roof fall. Risk estimates indicate that there is a very low probability of any of these scenarios occurring provided that the salt cavern is properly designed and operated.
A properly designed salt cavern can be a leak-free repository for NORM waste. Nevertheless, the present inability to control the flow of water within a subterranean salt formation during solution mining has made it difficult for the petroleum industry to convert salt cavern design concepts into reality in the field. A need, therefore, exists for a new method for forming salt caverns in a controlled manner and then depositing NORM wastes therein.